How do I convert a Mesh's autosmooth result into an edge selection?
It probably isn't possible to convert the selection Auto Smooth uses internally to define edges shown as sharp ones however it should be easy to select those edges additionally. In the 3d viewport header open Select > Sharp Edges to run the operator and create the selection.
In the operator options make the angle threshold to be the same as in the Auto Smooth option (so if Auto Smooth was set to 30 degrees make sure same angle is set in the Sharp Edges selection options). In the end same edges as ones used by Auto Smooth will become selected.